The present invention relates generally to a control system for a fission-type nuclear reactor, and more particularly to a control system for a fission-type nuclear reactor which utilizes a gas having a high neutron cross section to control the core reactivity.
A typical fission-type nuclear reactor includes a core comprised of the reactor fuel and a control system for controlling the reaction rate, or reactivity, of the reactor fuel in the core. The reactor fuel is typically an isotope of uranium, such as uranium 235, but may also comprise other suitable fuels. The reactor fuel may take the form of a fluid, such as an aqueous solution of enriched uranium, but usually the fuel is solid, either metallic uranium or a ceramic such as uranium oxide or uranium plutonium oxide. The solid fuel material is typically fabricated into various plates, pellets, pins, etc., which are clustered into an assemblage called a fuel element. These fuel elements are arranged in a matrix in the core.
The system for controlling the reactivity of the reactor core is constructed of control rods, which are also arranged in matrix form. The control rods are generally made of a neutron-absorbing material, such as boron carbide or some other neutron "poison" material. The poison material absorbs some of the neutrons emitted from the fuel elements which are necessary for sustaining the fission reaction.
The control rods are inserted and retracted into the core relative to the fuel elements to control the fission reaction rate. The fission rate in the reactor is increased by retracting the control rods from the core and decreased by inserting them into the core.
The control rods are typically inserted and retracted by an electromechanically driven mechanism, which uses a number of moving components. For example, one popular system includes a magnetically actuated control-rod drive which moves an extension of the control rod by alternatively moving one or the other of two latches. Each latch is magnetically operated by two coils. One of the coils operates to cause the latch to engage the control-rod extension and the other operates to raise the latch once it is engaged. The raising latch then raises the control rod with it. Another latch then engages, the first latch disengages, and the rod is raised by the second latch. Accordingly, through this sequence of coil actuations, the latches function in a hand-over-hand manner, retracting or inserting a control rod.
Although these electromechanical mechanisms are somewhat effective, as can be seen from the above description of their operation, their electromechanical nature requires a number of moving components which must accurately cooperate with each other. The number of moving parts presents a problem with the reliability and longevity of these systems. The reliability of the entire system depends on the reliability of the individual components.
In addition to the problems associated with the reliability and complicated nature of the control rods and associated drive mechanisms, these electromechanical control systems have other disadvantages. For example, if the control rods are only partially inserted to achieve a desired reactivity, the flux in the core will be distorted due to this partial insertion. This flux distortion results in lower power density of the core. Burnable poison rods are typically used to adjust the flux in the core.
Reactor control systems which do not use electromechanical components have also been proposed. For example, one proposed reactor control system involves the use of a liquid with a high neutron cross-section or neutron "poison". This system is commonly referred to as a chemical shim control system. The liquid passes through the coolant system channels or through dedicated tubes which are arranged throughout the reactor core. In this proposed system, the reactivity worth of the tubes is adjusted by varying the concentration of the neutron poison in each individual tube. To reduce the reactivity in a particular section of the core, the concentration of the neutron poison in the liquid in the tubes of the particular section of the core is increased. To increase the reactivity, the concentration of the neutron poison is decreased. Thus, this system allows the reactivity in each tube (or a group of tubes) to be controlled individually, providing greater accuracy in the control of the core.
Although this proposed liquid neutron poison system does not have as many moving parts as electromechanical system, it is not without disadvantages. For example, it may be difficult to quickly change the concentration of the neutron poison in the tubes. Changes in the concentration of the neutron poison may occur at a much slower rate than is desired. The time which is necessary to change the concentration reduces the response time of the reactor control system. Hence these systems are usually used in conjunction with electromechanical control rod systems.
U.S. Pat. No. 3,900,365 (Barclay et al.) discloses a reactor shut-down system which utilizes a liquid neutron poison. The system disclosed in this patent includes a plurality of tubes which communicate at one end with a reservoir containing a liquid neutron poison. The reservoir is arranged at an elevation above the reactor core providing hydrostatic pressure to bias the poison to flow into the reactor core. At the other end, the tubes are connected to a gas supply. A valve controls the gas pressure in each of the tubes. In order to shut down the reactor, the valve is used to lower the gas pressure in the tubes allowing the liquid neutron poison to flow upwardly through the tubes. The liquid neutron poison then decreases the reactivity in the core.
Although the system disclosed by Barclay et al. provides an efficient system to shut down a reactor, it does not provide a solution to the problems which are described above regarding the control of the reactivity of the core during normal operations. Changing the pressure in the tubes will either raise or lower the level of the liquid poison in the tubes. The lower part of the tube will be filled with a neutron poison, while the upper part of the tube will be filled with the gas. Therefore, use of this system to control the reactivity during normal operation of the reactor will not allow for radial flux shaping and will result in the same distorted axial flux profile problems associated with the solid control rods.
Other liquid control systems are also disclosed for example in "Control of Nuclear Reactors and Power Plants", M. A. Schultz, McGraw Hill, 1961.
Therefore in view of the above it is an object of the present invention to provide a nuclear reactor control system which is simpler than the presently used electromechanical systems.
It is another object of the present invention to provide a nuclear reactor control system which will provide the ability to shape the radial and axial flux distributions in the core.
It is still another object of the present invention to provide a nuclear reactor control system which does not require the use of chemical shim or burnable poison rods.
It is still another object of the present invention to provide a nuclear reactor control system which allows the reactivity to be precisely adjusted or controlled during normal operation of the reactor and which also permits rapid shut-down of the reactor.
It is still a further object of the present invention to provide a reactor control system which eliminates flux distortions due to partially inserted rods.